The present invention relates to an electrophotographic image forming apparatus and more particularly to a structure for supporting, when a photoconductive element or image carrier and other image forming members are arranged in a cartridge removable from the body of an image forming apparatus, supporting the photoconductive element.
Conventional printers, copiers, facsimile apparatus or similar electrophotographic image forming apparatuses include one operable with a removable image forming cartridge. The image forming cartridge accommodates a photoconductive element or image carrier and various image forming members arranged around the photoconductive element and including a charger, a developing unit and a cleaning unit. The cartridge is removably mounted on the apparatus body. This kind of scheme is taught in, e.g., Japanese Patent Laid-Open Publication No. 5-134482 and extensively used to promote easy maintenance and accurate positioning between the members.
Assume that the photoconductive element of the above cartridge is implemented by a photoconductive drum. Then, a shaft included in the drum is passed through holes formed in the side walls of the cartridge. Subsequently, positioning members are brought into contact with the shaft in order to position the axis of rotation of the drum.
Today, there is an increasing demand for an image forming apparatus capable of producing high quality images. One of current major attempts is to reduce irregularity in the pitch of a low density image, or so-called banding, ascribable to unstable drive of the apparatus. Specifically, because a low density image is formed by a smaller total amount of toner than a high density image, the above irregularity renders the difference between portions with toner and portions without toner conspicuous. Reducing banding as far as possible is therefore particularly important when it comes to a low density image.
Although the conventional cartridge configuration promotes easy maintenance, it fails to reduce banding to a sufficient degree. Specifically, when use is made of the cartridge, a drive source for driving the drum and other image forming members of the cartridge is mounted on the apparatus body and therefore needs a drive transmission mechanism to the cartridge. Two different systems are available for the drive transmission from the drive source to the cartridge. In a first system, a drive force is transmitted from the drive source to the drum and to the other members arranged in the cartridge via the drum. In a second system, the drive source is transmitted to each of the drum and other members via a particular path.
Generally, the above first system includes a gear or drive transmitting means mounted on one axial end of the shaft of the drum and held in mesh with gears mounted on the other members to be driven. This system is, however, not feasible for the image forming devices to be driven via the drum, particularly the developing unit. A heavy loads acts on the developing unit because not only a toner supply section but also an agitating member for charging toner are driven. As a result, the drive load is apt to vary between the gear of the developing unit and that of the drum, rendering the rotation of the drum irregular. As for a low density image, the drum makes the density of a portion where it has rotated at a high speed lower than desired density and makes the density of a portion where it has rotated at a low speed higher than the desired density. As a result, the density pitch becomes irregular due to the irregular rotation.
The second system obviates the drive transmission paths extending to the heavy load sections by way of the drum. However, the problem with the second system is that drive loads vary on the drive transmission paths between the apparatus body and the image forming members other than the drum. The above variation is imparted to the cartridge as fine vibration, causing the drum to generate fine vibration within the apparatus body. As a result, the drum and an optical writing device disposed in the apparatus body are displaced relative to each other. Such displacement is no better than the irregular rotation of the drum and renders an image irregular.